I don't want to see Whedon kill a character just to do it. And any characters that have their own franchises are not going to be killed. Which leaves Hawkeye and Black Widow as potential victims. Instead of killing them off, why not just have them leave the team or get injured and forced to stay out of a fight.

Regarding a Planet Hulk film, I think you could have Hulk turn into Banner on the planet. I know it wasn't that way in the comics, but it would work in a film. It would also allow for quite a bit of drama, as Banner is far more vulnerable to all the crazy aliens. It would actually work well as an animated tv show, too. I know they did the one film, but it could be stretched out with Hulk visiting different places on the planet and meeting different cultures and societies, slowly gaining allies for an eventual face-off with the Red King.